Transmit/receive compensation

ABSTRACT

In a time division duplex (TDD) system, compensation measurements are made for the transmission circuitry during the receive portion of the TDD cycle and compensation measurements are made for the receive circuitry during the transmit portion of the TDD cycle. A base station has multiple antennas for spatial, as well as spectral, spreading and despreading of discrete multitone spread spectrum (DMT-SS) communications. Each antenna has its own transmission path components and receive path components. The transmit amplifier, for example, in the transmit path and the receive amplifier, for example, in the receive path tend to drift in their characteristics over time. The invention manages the sequential testing of each respective transmission path and receive path for each antenna. The invention measures the drift of the transmit path components and the receive path components and prepares compensating weights to be applied to signals processed in each path.

This is a continuation of Ser. No. 08/806,508, filed Feb. 24, 1997, nowU.S. Pat. No. 5,864,543.

CROSS-REFERENCES TO RELATED APPLICATIONS

The invention disclosed herein is related to the copending US patentapplication by Siavash Alamouti, Doug Stolarz, and Joel Becker, entitled“VERTICAL ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULTITONE SPREADSPECTRUM COMMUNICATIONS SYSTEM”, assigned to AT&T Wireless Services, andincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention involves improvements to communications systems andmethods in a wireless discrete multitone spread spectrum communicationssystem.

2. Description of Related Art

Wireless communications systems, such as cellular and personalcommunications systems, operate over limited spectral bandwidths. Theymust make highly efficient use of the scarce bandwidth resource toprovide good service to a large population of users. Code DivisionMultiple Access (CDMA) protocol has been used by wireless communicationssystems to efficiently make use of limited bandwidths. The protocol usesa unique code to distinguish each user's data signal from other users'data signals. Knowledge of the unique code with which any specificinformation is transmitted, permits the separation and reconstruction ofeach user's message at the receiving end of the communication channel.

The personal wireless access network (PWAN) system described in thereferenced Alamouti, et al. patent application, uses a form of the CDMAprotocol known as discrete multitone spread spectrum (DMT-SS) to provideefficient communications between a base station and a plurality ofremote units.

In this protocol, the user's data signal is modulated by a set ofweighted discrete frequencies or tones. The weights are spreading codesthat distribute the data signal over many discrete tones covering abroad range of frequencies. The weights are complex numbers with thereal component acting to modulate the amplitude of a tone while thecomplex component of the weight acts to modulate the phase of the sametone. Each tone in the weighted tone set bears the same data signal.Plural users at the transmitting station can use the same tone set totransmit their data, but each of the users sharing the tone set has adifferent set of spreading codes. The weighted tone set for a particularuser is transmitted to the receiving station where it is processed withdespreading codes related to the user's spreading codes, to recover theuser's data signal. For each of the spatially separated antennas at thereceiver, the received multitone signals are transformed from timedomain signals to frequency domain signals. Despreading weights areassigned to each frequency component of the signals received by eachantenna element. The values of the despreading weights are combined withthe received signals to obtain an optimized approximation of individualtransmitted signals characterized by a particular multitone set andtransmitting location. The PWAN system has a total of 2560 discretetones (carriers) equally spaced in 8 MHZ of available bandwidth in therange of 1850 to 1990 MHZ. The spacing between the tones is 3.125 kHz.The total set of tones are numbered consecutively form 0 to 2559starting from the lowest frequency tone. The tones are used to carrytraffic messages and overhead messages between the base station and theplurality of remote units. The traffic tones are divided into 32 trafficpartitions, with each traffic channel requiring at least one trafficpartition of 72 tones.

In addition, the PWAN system uses overhead tones to establishsynchronization and to pass control information between the base stationand the remote units. A Common Link Channel (CLC) is used by the base totransmit control information to the Remote Units. A Common AccessChannel (CAC) is used to transmit messages from the Remote Unit to theBase. There is one grouping of tones assigned to each channel. Theseoverhead channels are used in common by all of the remote units whenthey are exchanging control messages with the base station.

In the PWAN system, Time Division Duplexing (TDD) is used by the basestation and the remote unit to transmit data and control information inboth directions over the same multi-tone frequency channel. Transmissionfrom the base station to the remote unit is called forward transmissionand transmission from the remote unit to the base station is calledreverse transmission. The time between recurrent transmissions fromeither the remote unit or the base station is the TDD period. In everyTDD period, there are four consecutive transmission bursts in eachdirection. Data is transmitted in each burst using multiple tones. Thebase station and each remote unit must synchronize and conform to theTDD timing structure and both the base station and the remote unit mustsynchronize to a framing structure. All remote units and base stationsmust be synchronized so that all remote units transmit at the same timeand then all base stations transmit at the same time. When a remote unitinitially powers up, it acquires synchronization from the base stationso that it can exchange control and traffic messages within theprescribed TDD time format. The remote unit must also acquire frequencyand phase synchronization for the DMT-SS signals so that the remote isoperating at the same frequency and phase as the base station.

The PWAN wireless communications system, and other limited bandwidthcommunications systems, need to exploit new techniques to make the mostefficient use of the scarce bandwidth resource to provide good serviceto a large population of users. When the characteristics of the transmitpath and the receive path change with time, some means is required tocompensate for tha drift without imposing additional overhead on thetraffic bearing channels.

SUMMARY OF THE INVENTION

The invention disclosed herein is a new technique to make the mostefficient use of the scarce spectral bandwidth. In a time divisionduplex (TDD) system, compensation measurements are made for thetransmission circuitry during the receive portion of the TDD cycle andcompensation measurements are made for the receive circuitry during thetransmit portion of the TDD cycle. A base station has multiple antennasfor spatial, as well as spectral, spreading and despreading of discretemultitone spread spectrum (DMT-SS) communications. Each antenna has itsown transmission path components and receive path components. Thetransmit amplifier, for example, in the transmit path and the receiveamplifier, for example, in the receive path tend to drift in theircharacteristics over time. The invention manages the sequential testingof each respective transmission path and receive path for each antenna.The invention measures the drift of the transmit path components and thereceive path components and prepares compensating weights to be appliedto signals processed in each path.

The base station's digital signal processor (DSP) applies the spreadingand despreading weights for the DMT-SS signals for the transmit path andthe receive path, respectively, for each antenna. In a first TDDinterval, a test controller coupled to the DSP, uses the TDD timingsignal from the DSP to first test the receive path of a first antenna(during the base station transmission period). To test the receive path,the test controller takes a multitone test signal output from thefrequency modulator in the transmit path and applies it to a testtransmitter that directs the multitone signal to the input of thereceive amplifier in the receive path. The DSP processes the receivedtest signal output by the receive amplifier and compiles receive pathcompensation weights that are stored in a receive path compensationbuffer. The receive path compensation weights are then applied to theDMT-SS signals received in all later TDD receive periods, until thereceive path test for that antenna are repeated. In one embodiment ofthe invention, a switch under the control of the test controllerselectively directs the multitone test signal output from the testtransmitter to the input of the receive amplifier in the receive path.In another, preferred embodiment of the invention, a probe antennacoupled to the output of the test transmitter directs the multitone testsignal output from the test transmitter to the input of the receiveamplifier in the receive path.

In the base receive period of the first TDD interval, the testcontroller coupled to the DSP, uses the TDD timing signal from the DSPto test the transmission path of the first antenna. To test thetransmission path, the test controller applies a multitone test signaloutput from the frequency modulator in the transmit path to thetransmitter in the transmit path. The test controller then directs theresultant signal output from the transmitter in the transmit path to theinput of a test receiver. The output of the test receiver is thanapplied to the multitone frequency demodulator in the receive path, in ashort interval so as to not overlap the DMT-SS signals being output bythe receive amplifier during the receive period. The DSP processes thereceived test signal applied by the test receiver and compiles transmitpath compensation weights that are stored in a transmit pathcompensation buffer. The transmit path compensation weights are thenapplied to the DMT-SS signals transmitted in all later TDD receiveperiods, until the transmit path test for that antenna is repeated. Inone embodiment of the invention, a switch under the control of the testcontroller selectively directs the signal output from the transmitteramplifier to the input of the test receiver. In another, preferredembodiment of the invention, a probe antenna coupled to the output ofthe transmitter amplifier directs the signal output from the transmitteramplifier to the input of the test receiver.

The test controller then moves on to the second antenna in the nextconsecutive (second) TDD interval. The test controller coupled to theDSP, uses the TDD timing signal from the DSP to first test the receivepath of a second antenna (during base station transmission period) andthen to test the transmit path of the second antenna (during basestation receive period).

Currently, the invention has advantageous applications in the field ofwireless communications, such as cellular communications or personalcommunications, where bandwidth is scarce compared to the number of theusers and their needs. Such applications may be effected in mobile,fixed, or minimally mobile systems. However, the invention may beadvantageously applied to other, non-wireless, communications systems aswell.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram showing an overview of transmit/receivecompensation according the present invention.

FIG. 1A is a block diagram showing an overview of transmit/receivecompensation according to an alternate embodiment of the presentinvention.

FIG. 1B is a block diagram showing an overview of transmit/receivecompensation according to another alternate embodiment of the presentinvention.

FIG. 2 is a timing diagram for transmit/receive compensation timing fora base station;

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a block diagram showing an overview of transmit/receivecompensation according the present invention. FIG. 2 is a timing diagramfor transmit/receive compensation timing for a base station. In a timedivision duplex (TDD) system, compensation measurements are made for thetransmission circuitry during the receive portion of the TDD cycle andcompensation measurements are made for the receive circuitry during thetransmit portion of the TDD cycle. A base station 100 has multipleantennas 115 for spatial, as well as spectral, spreading and despreadingof discrete multitone spread spectrum (DMT-SS) communications. A basestation 100 has a plurality of N transmit/receive modules, for examplethe four modules 102, 103, 104, and 105 in FIG. 1. Each transmit/receivemodule has an antenna 115 coupled to a transmit/receive switch 106. Eachtransmit/receive switch 106 is controlled to be in the transmit modewhen the transmit enable signal is on, and is controlled to be in thereceive mode when the receive enable signal is on. Antenna 1 is ontransmit/receive module 102. Antenna 2 is on transmit/receive module103. Antenna 3 is on transmit/receive module 104. Antenna 4 is ontransmit/receive module 105. Each antenna 115 has its own transmissionpath components and receive path components. In FIG. 1, the transmissionpath includes the transmit compensation weight buffer 111, the packetforming buffer 112, the multitone frequency modulator 113, and thetransmitter 114. The receive path includes the receiver 107, themultitone frequency demodulator 108, the packet buffer 109, and thereceive compensation weight buffer 110. The transmit compensation weightbuffer 111 and the receive compensation weight buffer 110 in eachtransmit/receive module are coupled to a digital signal processor (DSP)101 common to all of the transmit/receive modules. The receiver 107 andthe transmitter 114 in each transmit/receive module is coupled to thetransmit/receive switch 106 on the same transmit/receive module.

The transmitter amplifier 114 in the transmit path and the receiveramplifier 107 in the receive path tend to drift in their characteristicsover time. The invention manages the sequential testing of eachrespective transmission path and receive path for each antenna 115. Theinvention measures the drift of the transmit path components and thereceive path components and prepares compensating weights to be appliedto signals processed in each path. The sequential testing isaccomplished by the test controller 116, the test transmitter 117, andthe test receiver 118 that are common to all of the plurality oftransmit/receive modules 102, 103, 104, and 105 in FIG. 1. The testcontroller 116 receives time division duplex (TDD) timing signals fromthe DSP 101 and also receives the receive enable and the transmit enablesignals for timing. The test transmitter 117 is sequentially coupled tothe receiver 107 on each of the plurality of transmit/receive modules102, 103, 104, and 105 by means of the switch 119 controlled by the testcontroller 116. The test transmitter 117 is sequentially coupled to themultitone frequency modulator 113 on each of the plurality oftransmit/receive modules 102, 103, 104, and 105 by means of the switch121 controlled by the test controller 116. The test receiver 118 issequentially coupled to the transmitter 114 on each of the plurality oftransmit/receive modules 102, 103, 104, and 105 by means of the switch120 controlled by the test controller 116. The test receiver 118 issequentially coupled to the multitone frequency demodulator 108 on eachof the plurality of transmit/receive modules 102, 103, 104, and 105 bymeans of the switch 122 controlled by the test controller 116.

The base station's digital signal processor (DSP) 101 applies thespreading and despreading weights for the DMT-SS signals for thetransmit path and the receive path, respectively, for each antenna 115.In a first TDD interval, a test controller 116 coupled to the DSP, usesthe TDD timing signal from the DSP to first test the receive path of afirst antenna 1 on transmit/receive module 102 (during the base stationTDD transmission period). To test the receive path, the test controller116 takes a multitone test signal output from the frequency modulator113 in the transmit path and applies it to a test transmitter 117 thatdirects the multitone signal to the input of the receive amplifier 107in the receive path. The DSP 101 processes the received test signaloutput by the receive amplifier 107 and compiles receive pathcompensation weights that are stored in a receive path compensationbuffer 110. The receive path compensation weights are then applied bythe DSP 101 to the DMT-SS signals received in all later TDD receiveperiods, until the receive path test for that antenna 1 on module 102 isrepeated.

In the base station's receive period of the first TDD interval, the testcontroller 116 coupled to the DSP 101, uses the TDD timing signal fromthe DSP to test the transmission path of the first antenna 1 of module102. To test the transmission path, the test controller 116 applies amultitone test signal output from the frequency modulator 113 in thetransmit path to the transmitter 114 in the transmit path. The testcontroller 116 then directs the resultant signal output from thetransmitter 114 in the transmit path to the input of a test receiver118. The output of the test receiver 118 is than applied to themultitone frequency demodulator 108 in the receive path, in a shortinterval so as to not overlap the DMT-SS signals being output by thereceive amplifier 107 during the receive TDD period. The DSP 101processes the received test signal applied by the test receiver 118 andcompiles transmit path compensation weights that are stored in atransmit path compensation buffer 111. The transmit path compensationweights are then applied by the DSP 101 to the DMT-SS signalstransmitted in all later TDD receive periods, until the transmit pathtest for that antenna 1 on module 102 is repeated.

The test controller 116 then moves on to the second antenna 2 on module103 in the next consecutive (second) TDD interval by means of theswitches 119, 120, 121, and 122. The test controller 116 coupled to theDSP, uses the TDD timing signal from the DSP to first test the receivepath of a second antenna 2 of module 103 (during base stationtransmission TDD period) and then to test the transmit path of thesecond antenna 2 on module 103 (during base station receive TDD period).The new receive path compensation weights are then applied by the DSP101 to the DMT-SS signals received in all later TDD receive periods,until the receive path test for that antenna 2 on module 103 isrepeated. The new transmit path compensation weights are then applied bythe DSP 101 to the DMT-SS signals transmitted in all later TDD receiveperiods, until the transmit path test for that antenna 2 on module 103is repeated.

Then, the test controller 116 moves on to the third antenna 3 on module104 in the next consecutive (third) TDD interval by means of theswitches 119, 120, 121, and 122. The test controller 116 coupled to theDSP, uses the TDD timing signal from the DSP to first test the receivepath of the third antenna 3 of module 104 (during base stationtransmission TDD period) and then to test the transmit path of the thirdantenna 3 on module 104 (during base station receive TDD period). Thenew receive path compensation weights are then applied by the DSP 101 tothe DMT-SS signals received in all later TDD receive periods, until thereceive path test for that antenna 3 on module 104 is repeated. The newtransmit path compensation weights are then applied by the DSP 101 tothe DMT-SS signals transmitted in all later TDD receive periods, untilthe transmit path test for that antenna 3 on module 104 is repeated.

Finally, the test controller 116 moves on to the fourth antenna 4 onmodule 105 in the next consecutive (fourth) TDD interval by means of theswitches 119, 120, 121, and 122. The test controller 116 coupled to theDSP, uses the TDD timing signal from the DSP to first test the receivepath of the fourth antenna 4 of module 105 (during base stationtransmission TDD period) and then to test the transmit path of thefourth antenna 4 on module 105 (during base station receive TDD period).The new receive path compensation weights are then applied by the DSP101 to the DMT-SS signals received in all later TDD receive periods,until the receive path test for that antenna 4 on module 105 isrepeated. The new transmit path compensation weights are then applied bythe DSP 101 to the DMT-SS signals transmitted in all later TDD receiveperiods, until the transmit path test for that antenna 4 on module 105is repeated.

After completing transmit path and receive path compensation for allfour of the antennas on all four of the plurality of transmit/receivemodules 102, 103, 104, and 105 in FIG. 1, the test controller 116 beginsthe cycle again by testing the first antenna 1 on module 102.

In the first embodiment of the invention shown in FIG. 1 and describedabove, the switch 119 under the control of the test controller 116selectively directs the multitone test signal output from the testtransmitter 117 to the input of the receive amplifier 107, to test thereceive path. In an alternate, preferred embodiment of the inventionshown in FIG. 1A, a probe antenna 150T is connected to the output of thetest transmitter 117 and directs a controlled, low power version of themultitone test signal output from the test transmitter 117 to the inputof the receive amplifier 107 in the receive path. The controlled, lowpower version of the multitone test signal output can be received by theantenna 115, when it is appropriately switched to receive mode by switch106. Alternately, as is shown in the alternate embodiment of FIG. 1B, aprobe antenna 160R is connected to the input of the receiver amplifier107 to receive the controlled, low power version of the multitone testsignal transmitted by the probe antenna 115.

In the first embodiment of the invention shown in FIG. 1 and describedabove, the switch 120 under the control of the test controller 116selectively directs the test signal output from the transmitteramplifier 114 to the input of the test receiver 118, to test thetransmit path. In an alternate, preferred embodiment of the inventionshown in FIG. 1A, a probe antenna 150R is connected to the input of thetest receiver 118 and receives the test signal output from thetransmitter amplifier 114 to the input of the test receiver 118. Acontrolled, low power version of the test signal output by transmitteramplifier 114 can be transmitted by the anten;; 115, when it isappropriately switched to transmit mode by switch 106. Alternately, asis shown in the alternate embodiment of FIG. 1B, a probe antenna 160T isconnected to the output of the transmitter amplifier 114 to transmit thecontrolled, low power version of the test signal output by thetransmitter amplifier 114 to the probe antenna 150R input to the testreceiver 118.

The receive compensation weights and the transmit compensation weightscompensate for any differences in the characteristics of the two paths.The retro directivity principle described for the PWAN system relies onthe condition that the characteristics of the transmit and receive pathsare identical. However, because of component drift and other changeswith time in the two paths, their characteristics are not identical. Inaccordance with the invention, the receive compensation weights and thetransmit compensation weights are used by the DSP to compensate for anydifferences in the characteristics of the two paths. This brings thetransfer function of the circuitry in the transmit path and the receivepath more nearly the same. The compensation weights may be altered inonly the transmit path, to bring the characteristics of the transmitpath into equality with the characteristics of the receive path. The setof compensating weights can be applied to the transmitted data so thatat the antenna, the transmit (forward) and receive (reverse) path lookidentical.

In the PWAN system, the de-spreading weights at the receiving node (thebase station) are with minor modification used as spreading weights ontransmission. If the link media were truly identical, there would beidentical weights in both paths. The problem is that either throughdrift, tolerances in the electronics, and other real life variations thetwo links are not absolutely identical. In accordance with theinvention, the compensation weights are applied by the DSP as anadditive factor to the DMT-SS despreading and the spreading weights, tomake the effect of the de-spreading weights and the spreading weightsthe same.

In TDD receive mode at the base station, the transmit path is idle. Thetransmit path compensation measurement is performed by sending out apredetermined set of tones through the transmitter 114. This is receivedby the probe antenna 150R in FIG. 1A and demodulated and investigated.This gives a measurement of the base station's transfer function fromthe transmitter 114 to the probe antenna 150R and the test receiver 118.The probe antenna 150R is positioned near the antenna 115 on the basestation. The point on the path between the each of the plurality ofantennas 115 and the probe antenna 150R should be phase matched. Thesame can be said for the probe antenna 150T, for its position near theantenna 115 on the base station. In an alternate embodiment, the probeantenna 150R and the probe antenna 150T can be the same physicalantenna, suitably switched in its connection to the test receiver 118and the test transmitter 117, respectively, by the test controller 116.

The compensation measurements of the receive and transmit circuitry aremade during the idle time of the circuitry. The time division duplexingof the airlink results in a 50% duty cycle for the utilization of thetransmit and receive circuits. Therefore, compensation measurements canbe performed with the circuitry when the airlink does not require theiruse.

Use of the T/R duty cycle of the forward and reverse circuits to makeT/R compensation measurements frees system bandwidth and provides muchgreater measurement flexibility.

The personal wireless access network (PWAN) system described in thereferenced Alamouti, et al. patent application provides a more detaileddescription of the transmit path components and the receive pathcomponents alternately used in the TDD mode of operation. The basestation transmits information to multiple remote stations (or remoteunits) in its cell. The transmission formats are for a 64 kbits/sectraffic channel, together with a 4 kbps Link Control Channel (LCC)between the Base and a Remote Unit. The binary source delivers data tothe sender's transmitter at 64 kbits/sec. This translates to 48 bits inone transmission burst. The information bits are encrypted according toa triple data encryption standard (DES) algorithm. The encrypted bitsare then randomized in a data randomization block. A bit to octalconversion block converts the randomized binary sequence into a sequenceof 3-bit symbols. The symbol sequence is converted into 16 symbolvectors. The term vector generally refers to a column vector which isgenerally complex. One symbol from the LCC is added to form a vector of17 symbols.

The 17-symbol vector is trellis encoded. The trellis encoding startswith the most significant symbol (first element of the vector) and iscontinued sequentially until the last element of the vector (the LCCsymbol). This process employs convolutional encoding that converts theinput symbol (an integer between 0 and 7) to another symbol (between 0and 15) and maps the encoded symbol to its corresponding 16QAM (or16PSK) signal constellation point. The output of the trellis encoder istherefore a vector of 17 elements where each element is signal withinthe set of 16 QAM (or 16PSK) constellation signals. (The term signalwill generally refer to a signal constellation point.)

A link maintenance pilot signal (LMP) is added to form an 18-signalvector, with the LMP as the first elements of the vector. The resulting(18×1) vector is pre-multiplied by a (18×18) forward smearing matrix toyield a (18×1) vector b.

Vector b is element-wise multiplied by the (18×1) gain preemphasisvector to yield another (18×1) vector, c, where p denotes the trafficchannel index and is an integer. Vector c is post-multiplied by a (1×32)forward spatial and spectral spreading vector to yield a (18×32) matrixR(p). The number 32 results from multiplying the spectral spreadingfactor 4 and spatial spreading factor 8. The 18×32 matricescorresponding to all traffic channels carried (on the same trafficpartition) are then combined (added) to produce the resulting 18×32matrix S.

The matrix S is partitioned (by groups of four columns) into eight(18×4) submatrices (A₀ to A₇). (The indices 0 to 7, corresponds to theantenna elements over which these symbols will eventually betransmitted.) Each submatrix is mapped to tones within one trafficpartition.

A lower physical layer places the baseband signals in discrete Fouriertransfer (DFT) frequency bins where the data is converted into the timedomain and sent to its corresponding antenna elements (0 to 7) fortransmission over the air.

This process is repeated from the start for the next 48 bits of binarydata to be transmitted in the next forward transmission burst.

The resulting invention compensates for drift in the characteristics ofthe transmit path and the receive path without imposing additionaloverhead on the traffic bearing channels.

Still another alternate embodiment applies the above described inventionin the PWAN Frequency Division Duplex Communications System described inthe Alamouti, Michaelson et al. patent application cited above.

Although the preferred embodiments of the invention have been describedin detail above, it will be apparent to those of ordinary skill in theart that obvious modifications may be made to the invention withoutdeparting from its spirit or essence. Consequently, the precedingdescription should be taken as illustrative and not restrictive, and thescope of the invention should be determined in view of the followingclaims.

What is claimed is:
 1. A highly bandwidth-efficient communicationsmethod, comprising the steps of: receiving in a receive path at a basestation during a first time period a first spread signal comprising afirst data signal spread over a plurality of discrete tones;compensating for drift in said receive path during said first timeperiod by applying receive compensation weights to said first spreadsignal; testing a transmit path at said base station during said firsttime period and compiling transmit compensation weights; spreading asecond data signal at the base station with a spreading code thatdistributes the second data signal over a plurality of discrete tonesduring a second time period; applying said transmit compensation weightsto said second data signal during said second time period; transmittingsaid second spread signal during said second time period; and testingsaid receive path at said base station during said second time periodand compiling new receive compensation weights.
 2. In the highlybandwidth-efficient communications method of claim 1, wherein said firstand second time periods are part of a time division duplex interval. 3.In the highly bandwidth-efficient communications method of claim 1,wherein said compensation measurements are made for the transmissionpath during the receive portion of a time division duplex cycle andcompensation measurements are made for the receive path during thetransmit portion of the TDD cycle.
 4. In the highly bandwidth-efficientcommunications method of claim 1, wherein said base station has multipleantennas for spatial spreading and despreading of discrete multitonespread spectrum (DMT-SS) communications.
 5. In the highlybandwidth-efficient communications method of claim 4, wherein each saidantenna has its own transmission path components and receive pathcomponents.
 6. In the highly bandwidth-efficient communications methodof claim 5, wherein a transmit amplifier in the transmit path and areceive amplifier in the receive path tend to drift in theircharacteristics over time.
 7. In the highly bandwidth-efficientcommunications method of claim 5, wherein said compensation stepsfurther comprise: managing sequential testing of each respectivetransmission path and receive path for each antenna; measuring a driftof the transmit path components and the receive path components andpreparing compensating weights to be applied to signals processed ineach path.
 8. A highly bandwidth-efficient communications system,comprising: means for receiving in a receive path at a base stationduring a first time period a first spread signal comprising a first datasignal spread over a plurality of discrete tones; means for compensatingfor drift in said receive path during said first time period by applyingreceive compensation weights to said first spread signal; means fortesting a transmit path at said base station during said first timeperiod and compiling transmit compensation weights; means for spreadinga second data signal at the base station with a spreading code thatdistributes the second data signal over a plurality of discrete tonesduring a second time period; means for applying said transmitcompensation weights to said second data signal during said second timeperiod; means for transmitting said second spread signal during saidsecond time period; and means for testing said receive path at said basestation during said second time period and compiling new receivecompensation weights.
 9. In the highly bandwidth-efficientcommunications system of claim 8, wherein said first and second timeperiods are part of a time division duplex interval.
 10. In the highlybandwidth-efficient communications system of claim 8, wherein saidcompensation measurements are made for the transmission path during thereceive portion of a time division duplex cycle and compensationmeasurements are made for the receive path during the transmit portionof the TDD cycle.
 11. In the highly bandwidth-efficient communicationssystem of claim 8, wherein said base station has multiple antennas forspatial spreading and despreading of discrete multitone spread spectrum(DMT-SS) communications.
 12. In the highly bandwidth-efficientcommunications system of claim 11, wherein each said antenna has its owntransmission path components and receive path components.
 13. In thehighly bandwidth-efficient communications system of claim 12, wherein atransmit amplifier in the transmit path and a receive amplifier in thereceive path tend to drift in their characteristics over time.
 14. Inthe highly bandwidth-efficient communications system of claim 12, whichfurther comprises: means for managing sequential testing of eachrespective transmission path and receive path for each antenna; meansfor measuring a drift of the transmit path components and the receivepath components and preparing compensating weights to be applied tosignals processed in each path.